In high frequency communication systems typified by mobile phones, high-frequency filters are used to remove unnecessary signals other than signals in the frequency band used for communication. Acoustic wave resonators such as surface acoustic wave (SAW) resonators have been used for the high-frequency filters. In the SAW resonator, an Interdigital Transducer (IDT) including electrode fingers is formed on a piezoelectric substrate such as a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate.
It has been known that a dielectric film covering the IDT is provided, and the dielectric film in the edge regions corresponding to both edges of an overlap region in which the electrode fingers overlap is made to be thicker than the dielectric film in the central region as disclosed in Japanese Patent Publication Application No. 2013-544041 (Patent Document 1).
The technique disclosed in Patent Document 1 can reduce transverse-mode spurious. However, the frequency adjustment is not easy in the structure of Patent Document 1. For example, when a frequency adjusting film is formed on the dielectric film, the frequency adjusting film is not uniformly formed because of the level difference of the dielectric film and/or the level difference of the electrode fingers, and the transverse-mode spurious occurs.